Initial frequency synchronization mechanism

ABSTRACT

Method for acquiring frequency of a desired channel having a carrier frequency F MAIN , for a dynamic receiver frequency F MOBILE , from a starting frequency F START , in the presence of high power adjacent interfering channels, wherein the starting frequency F START  is shifted from F MAIN  by not more than a predetermined frequency gap ΔF, the method includes the steps of determining a first frequency boundary and a second frequency boundary, detecting channels within a filtering bandwidth, selecting a dominant channel from the detected channels, progressing the dynamic receiver frequency F MOBILE  towards the carrier frequency of the dominant channel, detecting when the step of progressing has exceeded one of the first frequency boundary and the second frequency boundary, restarting the step of detecting channels, from the other of the one of the first frequency boundary and the second frequency boundary, and repeating from the step of detecting channels.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 09/731,821, filed Dec. 8, 2000, now U.S. Pat. No.6,738,607, which is a continuation application of U.S. patentapplication Ser. No. 09/012,361, filed Jan. 23, 1998, now U.S. Pat. No.6,175,722 issued Jan. 16, 2001.

FIELD OF THE INVENTION

The present invention relates to frequency acquisition in general and tofrequency acquisition in the presence of high power adjacent channels,in particular.

BACKGROUND OF THE INVENTION

Reference is now made to FIGS. 1A and 1B. FIG. 1A is a schematicillustration of frequency versus power, describing the initial stage ofan initial frequency synchronization procedure, known in the art. Thepresent example describes a closed loop automatic frequency control(AFC) mechanism.

FIG. 1B is a schematic illustration of frequency versus power,describing the final stage of the initial frequency synchronizationprocedure of FIG. 1A.

Arrow 14 represents the frequency of a mobile unit which detects andattempts to lock and synchronize with the carrier frequency 10 of a baseunit transmitter having a value of F_(BASE), which is located near by.In the present example the mobile unit further detects a carrierfrequency 12 provided by a neighbor transmitter, having a value ofF_(NEIGHBOR). The value of the mobile unit F⁰ _(MOBILE) is locatedbetween the values of the base unit frequency F_(BASE) and the neighbormobile transmitter frequency F_(NEIGHBOR).

In the present example the mobile unit 14 detects the signals providedby base 10 and the neighbor 12 wherein the received power of theneighbor 12 is higher than the received power of the base unit 10.

According to conventional initial synchronization procedures, the mobileunit frequency is synchronized with the frequency having the highestreceived power, which in the present example is the neighbor frequency12.

It will be noted that often the received frequencies are filtered so asto exclude undesired signals. Such a filter is represented by arc 16.These techniques often fail when the power of the undesired signal issignificantly high.

Accordingly the synchronization mechanism of the mobile unit setssynchronization path towards the neighbor frequency F_(NEIGHBOR) andstarts progressing its frequency 14 towards F_(NEIGHBOR). Finally thesynchronization mechanism allows the frequency of the mobile unit 14 toacquire and synchronize with the frequency of the neighbor unit 12. Thisis shown in FIG. 1B by aligning line 12 and arrow 14. As can be seen, atthis stage the frequency 10 of the base transmitter is filtered out bythe filter 16.

A conventional synchronization mechanism provides frequency shiftswithin a limited range, determined by its structure, such as VCO voltageand the like. It will be appreciated by those skilled in the art thatthe F_(NEIGHBOR) can be located outside this range in such a case,F_(MOBILE), might get stuck at the boundary frequency value which isclosest to F_(NEIGHBOR).

It will be appreciated by those skilled in the art that such situations,where the frequency of the mobile unit 14 is synchronized with thefrequency of neighbor unit 12 instead of the frequency of the base unit10, is not acceptable.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereference numerals indicate corresponding, analogous or similarelements, and in which:

FIG. 1A is a schematic illustration of frequency versus power,describing the initial stage of a conventional initial frequencysynchronization procedure;

FIG. 1B is a schematic illustration of frequency power, describing thefinal stage of the initial frequency synchronization procedure of FIG.1A;

FIG. 2A is a schematic illustration of frequency versus power,describing the initial stage of a frequency synchronization procedure,operative in accordance with the present invention;

FIG. 2B is a schematic illustration of frequency versus power,describing the secondary stage of a frequency synchronization procedure,operative in accordance with the present invention;

FIG. 2C is a schematic illustration of frequency versus power,describing the third stage of a frequency synchronization procedure,operative in accordance with the present invention;

FIG. 2D is a schematic illustration of frequency versus power,describing the final stage of a frequency synchronization procedure,operative in accordance with the present invention;

FIG. 2E is a schematic illustration of frequency versus power,describing the third stage of a frequency synchronization procedure,operative in accordance with another aspect of the present invention;

FIG. 2F is a schematic illustration of frequency versus power,describing the final stage of a frequency synchronization procedure,operative in accordance with another aspect of the present invention;

FIG. 3 is a schematic illustration of a device for synchronizingfrequencies, constructed and operative in accordance with anotherpreferred embodiment of the invention;

FIG. 4 is a schematic illustration of a method for operating the deviceof FIG. 3, operative in accordance with a further embodiment of theinvention;

FIG. 5A is a schematic illustration of a method for operating the deviceof FIG. 3, operative in accordance with yet another embodiment of theinvention; and

FIG. 5B is a schematic illustration in detail of a step of the method ofFIG. 5A.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those of ordinary skill in the artthat the present invention may be practiced without these specificdetails. In other instances, well-known methods, procedures, componentsand circuits have not been described in detail so as not to obscure thepresent invention.

The present invention overcomes the disadvantages of the prior art byproviding a frequency detect and fold mechanism. Accordingly, when thefrequency shift exceeds a boundary value, then a predetermined frequencyshift is enforced.

Reference is now made to FIGS. 2A, 2B, 2C and 2D. FIG. 2A is a schematicillustration of frequency versus power, describing the initial stage ofa frequency synchronization procedure, operative in accordance with thepresent invention FIG. 2B is a schematic illustration of frequencyversus power, describing the secondary stage of a frequencysynchronization procedure, operative in accordance with the presentinvention. FIG. 2C is a schematic illustration of frequency versuspower, describing the third stage of a frequency synchronizationprocedure, operative in accordance with the present invention. FIG. 2Dis a schematic illustration of frequency versus power, describing thefinal stage of a frequency synchronization procedure, operative inaccordance with the present invention.

The schematic illustration provided by FIG. 2A describes the frequency100 of a base station transmitting a desired signal, having a valueF_(BASE), a frequency 104 of a mobile unit, having an initial value F⁰_(MOBILE), and a frequency 102 of a neighbor transmitter, having thevalue of F_(NEIGHBOR), whereinF_(BASE)<F⁰ _(MOBILE)<F_(NEIGHBOR).

In conventional communication standards, such as AMPS, NAMPS, JTACS,NTACS, USDC-TDMA and the like, the initial value of F⁰ _(MOBILE) of themobile unit frequency 104 can be shifted from the value F_(BASE) of thebase station frequency 100, by no more than a predetermined frequencygap ΔF. Another condition set by these standards is that any neighbortransmitter will transmit in a frequency F_(NEIGHBOR), which isconsiderably shifted from F_(BASE). Accordingly|F_(BASE)−F_(NEIGHBOR)|>2ΔF.

The method of the present invention generally searches the receivedspectrum within a frequency range of [F⁰ _(MOBILE)−ΔF, F⁰ _(MOBILE)+ΔF],for stabilized frequency values.

According to the invention at the initial stage (i.e., at frequency F⁰_(MOBILE)) the mobile unit detects all of the signals of transmitters inits vicinity and detects the frequency of the signal with the highestreceived power, which in the present example is the neighbor transmittedfrequency 102. Accordingly, the mobile unit commences shifting itsfrequency 104 from the value of F⁰ _(MOBILE), towards the valueF_(NEIGHBOR) of neighbor transmitter frequency 102.

The present invention makes use of the above limitations, ofconventional communication standards, which outline that the initialvalue F⁰ _(MOBILE) of the mobile unit frequency 104 has to be within afrequency gap of ΔF from the value F_(BASE), of the base transmitterfrequency 100.

Accordingly, any shift from the initial stage F⁰ _(MOBILE), cannotexceed the value of ΔF. After the frequency 104 of the mobile unit hasprogressed towards the neighbor transmitter frequency 102 valueF_(NEIGHBOR), by a frequency shift 110, having a value of ΔF, to thevalue F¹ _(MOBILE), then, according to the invention, any furtherprogress in this direction would result in a detection error and hence,should not be pursued.

At this stage, the present invention determines a reversed path 112 forfrequency 104 (FIG. 2C) for shifting frequency 104 from the value of F¹_(MOBILE) to the value of F² _(MOBILE) wherein the shift value of thisreverse path 112, is a frequency gap which is twice the value of ΔF.

At the final stage (FIG. 2D) the spectrum is searched, thereby detectingthe base frequency 100 as the dominant signal. Accordingly, the mobileunit 104 commences shifting its frequency towards base frequency 100,from the value of F² _(MOBILE) to F_(BASE). This shift is shown by path114. According to the present example, no direction is enforced for path114.

It will be noted that applying a filter, such as filter 106, improvesthe performance of an initial synchronization process, according to theinvention. As illustrated in FIG. 2C, as long as the filter size is lessthan |F_(BASE)−F_(NEIGHBOR)|×2, (provided that the filter is generallysymmetrical), wherein F_(NEIGHBOR) is not a high power signal, then,F_(NEIGHBOR) would not be detected as a major signal by the receiver ofthe mobile unit, in the original direction of progress.

Reference is now made to FIG. 3 which is a schematic illustration of adevice for synchronizing frequencies, generally referenced 200,constructed and operative in accordance with another preferredembodiment of the invention.

Device 200 includes a frequency shift unit 202, an intermediatefrequency (IF) filter 204 connected to the frequency shift unit 202, afrequency shift detector 206 connected to the IF filter 204, a loopfilter 208 connected to the frequency shift detector 206, a non-linearcontroller 210 connected to the loop filter 208, and a voltage controloscillator (VCO) 212, connected to the non-linear controller 210 and tothe frequency shift unit 202. It will be noted that VCO 212 can bereplaced with any type of controlled oscillator.

The frequency shift unit 202 is further connected to an antenna 220. Thefrequency shift detector 206 is further connected to a host 222. Thehost 222 provides a reference frequency value to the frequency shiftdetector 206.

The antenna 220 detects frequency signals of neighbor transmitterswherein one of these detected frequency signals is transmitted by a basestation. The antenna 220 provides these received frequency signals tothe frequency shift unit 202. The VCO 212 generates a signal having afrequency and provides it to frequency shift unit 202.

Frequency shift unit 202 shifts frequencies received from antenna 220according to the frequency provided by the VCO and provides the resultsto the IF filter 204. The IF filter 204 filters some of thesefrequencies and provides the remaining ones to the frequency shiftdetector 206. The frequency shift detector 206 attempts to detect thefrequency shift of each of these shifted frequencies from the referencefrequency value, provided by the host 222.

Accordingly, the frequency shift detector 206 determines a frequencyshift value and provides it to the loop filter 208. The loop filter 208stores information regarding the history of the frequency shiftsperformed by device 200 and accordingly determines a frequency shiftdirection and provides it with the frequency shift value to thenon-linear controller 210.

The non-linear controller 210 detects if the overall shift, up untilthis stage has exceeded the value of ΔF. If so, then the non-linearcontroller 210 provides VCO 212 with the command to generate a reversedfrequency shift such as the one according to path 112 (FIG. 2C). If not,then the non-linear control 210 provides the VCO 212 with a frequencyshift value and a frequency shift direction for further shifting thefrequency towards the most dominant received frequency. Then the VCO 212provides a new shift frequency to the frequency shift unit 202 and theprocess is repeated from the beginning.

It will be noted that when using a slow loop filter, such as softwareimplemented loop filter, it would be difficult for such a loop filter toprocess a considerable shift such as the one defined by path 112, sincesuch shifts are compared to frequency behavior history containedtherein.

According to a further aspect of the invention when the non-linearcontroller 210 determines a 2ΔF shift, it also sends a clear commandback to the loop filter 208, thereby erasing the information regardingthe frequency shift history contained in the memory of loop filter 208.This operation enables the loop filter 208 to further processconsiderable frequency shifts.

It will be noted that the terms base, mobile and neighbor are presentedas a matter of convenience only. The present invention is applicable forany type of initial frequency acquisition in the presence of a highpower adjacent channel, wherein the base of the above example isassigned to a main transmitter emitting the desired signal, the mobileof the above example is assigned to a receiver and the neighbor of theabove example is assigned to an adjacent interfering transmitter.

It will be noted that each of the main transmitter, the adjacenttransmitter and the receiver may be implemented for a mobile unit, abase unit and the like.

Reference is now made to FIG. 4 which is a schematic illustration of amethod for operating the device 200 of FIG. 3, operative in accordancewith a further embodiment of the invention.

In step 300, the device 200 stores the value F⁰ of the internal initialfrequency F. F⁰ is used to determine, later on, the total amount ofshift from the initial frequency. It will be noted that for thispurpose, the device 200 can store and accumulate the values of the laterfrequency shifts, instead.

In step 302, the device 200 detects incoming frequency signals.

In step 304, the device 200 filters the incoming frequency signals,thereby obtaining selected frequencies.

In step 306, the device 200 determines a target frequency valueF_(TARGET), from the selected frequencies. In the present example (FIG.2A), the device 200 (FIG. 3) selects the right side signal 102(F_(NEIGHBOR)), as the target frequency F_(TARGET).

In step 308, the device 200 progresses the internal frequency F towardsthe target frequency F_(TARGET) by a predetermined frequency stepF_(STEP). It will be noted that F_(STEP) can be determined using a rangeof considerations, such as speed, accuracy and the like. In general,F_(STEP) is determined to be significantly smaller than ΔF, therebyyielding higher accuracy. It will further be noted that F_(STEP) can beinfinitesimal thereby yielding an analog like behavior.

In step 310, the device 200 detects if the internal frequency F wasshifted beyond a first frequency boundary represented by a gap of ΔF. Ifso, then the device 200 proceeds to step 312. Otherwise, the device 200proceeds to step 314.

In step 312, the device 200 reverses F by 2ΔF to shift the internalfrequency F to a second frequency boundary. In the present example (FIG.2C), reverse path 112, describes such a reverse shift, from the value ofF¹ _(MOBILE) to the value of F² _(MOBILE). Then, the device 200 repeatsthe steps of the above method, from step 302.

It will be noted that at this stage, signal 102 appears to be outside ofthe filtering bandwidth of filter 106, thereby leaving the base stationfrequency signal 100, the strongest, at the output of filter 106.Accordingly, the device 200 determines F_(BASE) as F_(TARGET).

In step 314, the device 200 detects if the internal frequency F issynchronized with the target frequency F_(TARGET). If so, then thedevice 200 has completed the initial frequency acquisition procedure andaccordingly, locks the frequency F (step 316). Otherwise, the device 200repeats the steps of the above method, from step 302.

The method of FIG. 4 overcomes a situation where there exists aninterfering neighbor frequency such as F_(NEIGHBOR) (reference numeral102) on one side of the spectrum.

In a situation where there exist interfering neighbor frequencies onboth sides of the base frequency F_(BASE), the present inventionprovides a slightly different solution, as will be disclosedhereinbelow.

Reference is now made to FIGS. 2E and 2F. FIG. 2E is a schematicillustration of frequency versus power, describing a stage of afrequency synchronization procedure, operative in accordance withanother aspect of the present invention. FIG. 2F is a schematicillustration of frequency versus power, describing a final stage of afrequency synchronization procedure, operative in accordance withanother aspect of the present invention.

According to the present example, there exists an additional neighborfrequency 120 having a value of F*_(NEIGHBOR), on the left side of thebase frequency 100 F_(BASE). When the mobile frequency completes the 2ΔFfrequency shift 112, additional neighbor frequency 120 falls within thefiltering bandwidth of filter 106, together with base frequency 100.

It will be noted that if, at the output of filter 106, the signal of theadditional neighbor frequency 120 appears to be stronger than the signalof the base frequency 100, then, according to the method of FIG. 3, themobile frequency 104 would be drawn towards the additional neighborfrequency 120.

According to another aspect of the present invention, the initialdirection set forth in the second stage (i.e., the direction offrequency shift 110, (FIG. 2B)), is stored. In the present example, thisdirection is from left to right.

Then, after the mobile frequency completes the 2ΔF frequency shift 112,the acquisition mechanism continues searching in that initial direction,only. It will be noted that such forced search direction provides anaccurate acquisition of the desired base frequency, in one or lesssearch cycle.

In a more detailed form, at the final stage (FIG. 2F) the spectrum issearched again in the direction set forth in the initial stage (i.e.,the direction of shift 110), thereby detecting the base frequency 100 asthe dominant signal. Accordingly, a path 122 is set towards basefrequency 100, for shifting mobile frequency 104 from the value of F²_(MOBILE) to F_(BASE).

It will be noted that the present invention provides a search shift stepwhich can be calibrated at each search stage. For example, on the onehand, in the presence of a powerful additional neighbor 120, frequencyshift 122 may include a large number of infinitesimal frequency shiftsteps. Otherwise, frequency shift 122 may include a small number oflarger frequency shift steps.

Reference is now made to FIGS. 5A and 5B. FIG. 5A is a schematicillustration of a method for operating the device 200 of FIG. 3,operative in accordance with yet another embodiment of the invention.FIG. 5B is a schematic illustration in detail of step 406 of the methodof FIG. 5A.

In step 400, the device 200 stores the value F⁰ of the internal initialfrequency F.

In step 402, the device 200 detects incoming frequency signals.

In step 404, the device 200 filters the incoming frequency signals,thereby obtaining selected frequencies.

In step 406, the device 200 determines frequency step F_(STEP) and afrequency advance direction, in a way which is described in detail inFIG. 5B.

In step 418, if the detection performed according to step 402 is thefirst detection in the current acquisition cycle, then the device 200proceeds to step 420. Otherwise, the device 200 proceeds to step 408.

In step 420, the device 200 determines an initial advance directionwhich will be constant during the present acquisition cycle, andproceeds to step 408.

In step 408, the device 200 progresses the internal frequency F byfrequency step F_(STEP), in the advance direction.

In step 410, the device 200 detects if the internal frequency F wasshifted beyond a gap of ΔF. If so, then the device 200 proceeds to step412. Otherwise, the device 200 proceeds to step 414.

In step 412, the device 200 reverses F by 2ΔF. In the present example(FIG. 2E), reverse path 112, describes such a reverse shift, from thevalue of F¹ _(MOBILE) to F² _(MOBILE). Then, the device 200 repeats thesteps of the above method, from step 402.

It will be noted that at this stage, additional neighbor frequencysignal 120 falls within the filtering bandwidth of filter 106, whichposes a problem if additional neighbor frequency signal 120 appearsstronger than the base station signal 100, at the output of filter 106.

Referring now to FIG. 5B, the device 200 determines a target frequencyvalue F_(TARGET) from the selected frequencies (step 430). In thepresent example, when the mobile frequency is at a value of F⁰ _(MOBILE)(FIG. 2A), the device 200 (FIG. 3) selects the right side signal 102(F_(NEIGHBOR)), as the target frequency F_(TARGET). Alternatively, whenthe mobile frequency is at a value of F² _(MOBILE) (FIG. 2E), the device200 (FIG. 3) selects the left side signal 120 (F*_(NEIGHBOR)), as thetarget frequency F_(TARGET).

In step 432, if the detection performed according to step 402 is thefirst detection in the current acquisition cycle, then, the device 200proceeds to step 440.

Otherwise, the device 200 proceeds to step 434.

In step 434, the device 200 determines an advance direction from themobile frequency value F and the target frequency value F_(TARGET).

In step 436, if the advance direction determined in step 434 is equal tothe initial advance direction, determined in step 420, then the device200 proceeds to step 440. Otherwise, the device 200 proceeds to step438. It will be noted that a situation where these directions are notequal occurs, for example, when a neighbor signal, such as the one ofadditional neighbor frequency 120, appears to be stronger than thesignal of the base frequency 100, at the output of the filter 106.

In step 440, the device 200 determines the frequency step F_(STEP)according to the position of F and F_(TARGET). In the present example,F_(STEP)≦|F−F_(TARGET)|.

In step 438, the device 200 determines the advance direction to be theinitial advance direction.

In step 442, the device 200 determines a relatively small value forfrequency step F_(STEP). It will be noted that, according to the presentexample, the size of F_(STEP) is smaller, compared to the size of ΔF.

Referring back to FIG. 5A, wherein if the device 200 detects if theinternal frequency F is synchronized with the target frequencyF_(TARGET) (step 414), then the device 200 proceeds to step 416 andlocks F. Otherwise, the device 200 repeats the steps of the abovemethod, from step 402.

Hence, the method of FIGS. 5A and 5B overcomes a situation where thereexist interfering neighbor frequencies such as F_(NEIGHBOR) (referencenumeral 102) and F*−_(NEIGHBOR) (reference numeral 120) on either sideof the F_(BASE).

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the spirit ofthe invention.

1. A cellular telephone mobile station comprising: an antenna; and afrequency synchronization device including at least: a frequency shiftdetector to detect a frequency difference between an internal frequencyand a received frequency from a cellular telephone base station withrespect to a reference frequency and to produce a frequency shift value;a non-linear controller to control an oscillator to generate saidinternal frequency in a range from a first frequency boundary to asecond frequency boundary, and to provide said frequency shift value tosaid oscillator so that said oscillator adjusts said internal frequencyaccording to said value; and a loop filter coupled to said frequencyshift detector to filter said frequency shift value and to storeinformation regarding previous frequency shifts and accordingly todetermine a frequency shift direction.
 2. The mobile station of claim 1,wherein said controller is able to determine a frequency step.
 3. Themobile station of claim 1 further comprising: a frequency shift unitcoupled to said oscillator.
 4. The mobile station of claim 1, whereinsaid controller generates a command to shift said internal frequency tosaid second frequency boundary when said internal frequency reaches saidfirst frequency boundary.
 5. The mobile station of claim 1, wherein saidcontroller generates a command to erase said information regardingprevious frequency shifts stored in said loop filter.
 6. The mobilestation of claim 1, wherein said controller generates a command to shiftsaid internal frequency to said first frequency boundary when saidinternal frequency reaches said second frequency boundary.
 7. The mobilestation of claim 6, wherein said controller further generates a commandto erase said information regarding previous frequency shifts stored insaid loop filter.
 8. A cellular telephone base station comprising: anantenna; and a frequency synchronization device including at least: afrequency shift detector to detect a frequency difference between aninternal frequency and a received frequency from a cellular telephonemobile station with respect to a reference frequency and to produce afrequency shift value; a non-linear controller to control an oscillatorto generate said internal frequency in a range from a first frequencyboundary to a second frequency boundary, and to provide said frequencyshift value to said oscillator so that said oscillator adjusts saidinternal frequency according to said value; and a loop filter coupled tosaid frequency shift detector to filter said frequency shift value andto store information regarding previous frequency shifts and accordinglyto determine a frequency shift direction.
 9. The base station of claim8, wherein said controller is to determine a frequency step.
 10. Thebase station of claim 8, further comprising: a frequency shift unitcoupled to said oscillator.
 11. The base station of claim 8, whereinsaid controller is to generate a command to shift said internalfrequency to said second frequency boundary when said internal frequencyreaches said first frequency boundary.
 12. The base station of claim 8,wherein said controller is to generate a command to erase saidinformation regarding previous frequency shifts stored in said loopfilter.
 13. The base station of claim 8, wherein said controller is togenerate a command to shift said internal frequency to said firstfrequency boundary when said internal frequency reaches said secondfrequency boundary.
 14. The base station of claim 13, wherein saidcontroller is to generate a command to erase said information regardingprevious frequency shifts stored in said loop filter.
 15. A cellulartelephone system comprising: a base station apparatus and a mobilestation apparatus to communicate bidirectionally over a channel, and aneighboring transmitter apparatus to transmit on an adjacent channel,wherein said mobile station apparatus comprises: a transmitter totransmit radio frequency signals to said base station apparatus; areceiver to receive radio frequency signals from said base stationapparatus; an intermediate frequency filter to substantially attenuate acarrier signal received from said neighboring transmitter apparatusrelative to a carrier signal received from said base station apparatus;a frequency shift detector to detect a frequency difference between aninternal frequency of said receiver and a received frequency from saidbase station apparatus with respect to a reference frequency and toproduce a frequency shift value; a non-linear controller to control anoscillator to generate said internal frequency in a range from a firstfrequency boundary to a second frequency boundary, and to provide saidfrequency shift value to said oscillator so that said oscillator adjustssaid internal frequency according to said value; and a loop filtercoupled to said frequency shift detector to filter said frequency shiftvalue and to store information regarding previous frequency shifts andaccordingly to determine a frequency shift direction.
 16. The system ofclaim 15, wherein said controller is to determine a frequency step. 17.The system of claim 15, wherein said mobile station apparatus furthercomprises: a frequency shift unit coupled to said oscillator.
 18. Thesystem of claim 15, wherein said controller is to generate a command toshift said internal frequency to said second frequency boundary whensaid internal frequency reaches said first frequency boundary.
 19. Thesystem of claim 15, wherein said controller is to generate a command toerase said information regarding previous frequency shifts stored insaid loop filter.
 20. The system of claim 15, wherein said controller isto generate a command to shift said internal frequency to said firstfrequency boundary when said internal frequency reaches said secondfrequency boundary.
 21. The system of claim 20, wherein said controlleris to generate a command to erase said information regarding previousfrequency shifts stored in said loop filter.
 22. A cellular telephonesystem comprising: a base station apparatus and a mobile stationapparatus to communicate bidirectionally over a channel, and aneighboring transmitter apparatus to transmit on an adjacent channel,wherein said base station apparatus comprises: a transmitter to transmitradio frequency signals to at least said mobile station apparatus; areceiver to receive radio frequency signals from at least said mobilestation apparatus; an intermediate frequency filter to substantiallyattenuate a carrier signal received from said neighboring transmitterapparatus relative to a carrier signal received from said mobile stationapparatus; a frequency shift detector to detect a frequency differencebetween an internal frequency of said receiver and a received frequencyfrom said mobile station apparatus with respect to a reference frequencyand to produce a frequency shift value; a non-linear controller tocontrol an oscillator to generate said internal frequency in a rangefrom a first frequency boundary to a second frequency boundary, and toprovide said frequency shift value to said oscillator so that saidoscillator adjusts said internal frequency according to said value; anda loop filter coupled to said frequency shift detector to filter saidfrequency shift value and to store information regarding previousfrequency shifts and accordingly to determine a frequency shiftdirection.
 23. The system of claim 22, wherein said controller is todetermine a frequency step.
 24. The system of claim 22, wherein saidbase station apparatus further comprises: a frequency shift unit coupledto said oscillator.
 25. The system of claim 22, wherein said controlleris to generate a command to shift said internal frequency to said secondfrequency boundary when said internal frequency reaches said firstfrequency boundary.
 26. The system of claim 22, wherein said controlleris to generate a command to erase said information regarding previousfrequency shifts stored in said loop filter.
 27. The system of claim 22,wherein said controller is to generate a command to shift said internalfrequency to said first frequency boundary when said internal frequencyreaches said second frequency boundary.
 28. The system of claim 22,wherein said controller is to generate a command to erase saidinformation regarding previous frequency shifts stored in said loopfilter.